1. Field of the Invention
The present invention relates to an moving speed detection apparatus and, more particularly, to an object moving speed detection apparatus, which is applied to an auto-focus photographing apparatus of a camera, for driving a taking lens to an in-focus position on the basis of a focus detection output, and detects a moving speed of an object so as to prevent defocusing caused by movement of the object in the optical axis direction of the taking lens.
2. Description of the Related Art
Conventionally, when an object moving in the optical axis direction of a taking lens is to be photographed, defocusing occurs due to movement of the object during a release time lag.
As a technique for preventing defocusing, for example, Published Unexamined Japanese Patent Application No. 63-159817 discloses a technique for performing a plurality of times of distance measurement operations in response to a first release signal to predict the position of an object at the beginning of exposure, and driving a taking lens. In a field other than cameras, as described in, e.g., Published Unexamined Japanese Patent Application No. 62-232571, a method of projecting infrared rays onto an object to be measured, and detecting a moving speed of the object to be measured on the basis of a signal reflected by the object has been proposed.
A conventional speed detection apparatus will be described below with reference to Published Unexamined Japanese Patent Application No. 63-159817 described above.
In FIG. 23, reference numeral 1 denotes an object; and 2 to 4, a distance measurement optical system, a light-emitting element driver, and a distance calculation circuit, which constitute a distance measurement apparatus.
When an infrared light-emitting diode (IRED) 2a included in the distance measurement optical system 2 is driven by the light-emitting element driver 3, light from the IRED 2a is projected onto the object 1 via a projection lens 2b. The light projected onto the object 1 is reflected by the object, is received by a light-receiving lens 2c, and is then focused on an optical signal currents I.sub.1 and I.sub.2 according to the incident position of the reflected signal light. The signal currents I.sub.1 and I.sub.2 are processed by the distance calculation circuit 4, thus obtaining a distance to the object 1.
In the speed detection apparatus, the above-mentioned distance measurement operation is repeated at predetermined time intervals in accordance with a timing circuit 5. After the distance measurement result is stored in a distance data memory 6, a positional displacement of the object 1 in a predetermined period of time is calculated, thereby detecting the moving speed of the object.
In order to discriminate a change in speed, the speed detection apparatus comprises a special-purpose function determination circuit 7 consisting of an order discrimination circuit 7a, a linear function determination circuit 7b, and a quadratic function determination circuit 7c, and also includes a distance prediction/calculation circuit 8 for predicting a distance to an object at a photographing timing (at the beginning of exposure), a control circuit 9 for controlling these circuits, and the like.
The conventional speed detection apparatus is effective when a distance measurement time is negligibly short, and a distance measurement result includes no error at all.
However, in practice, these factors must be taken into consideration, and the apparatus suffers from the following drawbacks. More specifically, in order to strictly obtain an order of a function of a moving speed of the object 1 on the basis of distance data, complicated circuits are required, and the apparatus becomes expensive. When calculations are performed in a software manner using a one-clip microcomputer (e.g., a CPU), its calculation time is not negligible, and it is impossible to detect a speed of an object moving at high speed, such as an automobile.
For these reasons, the speed detection apparatus requires a distance measurement apparatus which can minimize a distance measurement error, and can perform a distance measurement operation at high speed. However, electronic circuits inevitably suffer from noise, and it is not easy to manufacture an ideal distance measurement apparatus.
The present applicant has made a proposal for realizing a high-precision auto-focus operation based on a noise canceling effect by an integration (e.g., Published Unexamined Japanese Patent Application No. 63-132110 (corresponding to U.S. Pat. No. 4,855,585)). However, in a distance measurement system which causes an IRED to emit light a plurality of times, a time lag is prolonged, and this method is not suitable for the speed detection apparatus.
FIGS. 24A to 24C show a general speed detection operation employing the conventional distance measurement system.
More specifically, when it is difficult to assure precision by a single distance measurement operation, the distance measurement operation is performed a plurality of times, so that noise components randomly included in distance measurement results can be canceled. However, as shown in FIG. 24A, when a plurality of times (e.g., four times) of distance measurement operations are performed during a time interval (.alpha.), a time lag is caused accordingly. In addition, since an object distance also changes during the four distance measurement operations, this method is not effective for measuring a distance to a moving object.
During a distance calculation operation for calculating a precise distance measurement result on the basis of the four distance measurement results during a time interval (.beta.), a time lag is also caused accordingly.
After the time interval (.beta.), four distance measurement operations are performed during a time interval (.gamma.) similar to the time interval (.alpha.), and a distance calculation operation for obtaining a precise distance measurement result based on the four results is performed during a time interval (.delta.). Thereafter, when a moving speed of an object is to be calculated using these two calculation results, a time interval (.epsilon.) for a speed calculation operation is required. As a result, a very long speed detection time is necessary.
Since a distance to a moving object is measured, the advantage of repeating a distance measurement operation a plurality of times is lost due to a change in moving object during the distance measurement operations.
Therefore, a considerable difficulty is expected in high-precision distance measurement operation with a short time lag, and speed detection of an object according to the conventional concept.
When an object is moving at a constant speed, the conventional speed detection apparatus poses the following problems when it is applied to an auto-focus photographing apparatus for a camera.
As shown in FIG. 25, in a normal taking lens, the following relation is established among an object distance l, a focal length f.sub.L of the lens, and a distance (extension amount) K between a film and the lens: EQU K.multidot.l=f.sub.L.sup.2
That is, EQU K=f.sub.L.sup.2 .multidot.1/l
Therefore, assuming that an object moves by 1 m during a given shutter time lag, an increase in defocusing amount is larger in a case wherein the object is moved from a 2-m position to a 1-m position than in a case wherein the object is moved from a 3-m position to a 2-m position. For this reason, when speed detection of an object at a near distance is performed with the same time lag as that for an object at a far distance, an extension correction amount based on moving object prediction becomes too large, and an error is increased accordingly. In the worst case, an object may pass a camera position at the start timing of exposure, resulting in an unsuccessful photographing operation.
In a so-called active system for performing a distance measurement operation by causing an IRED to emit light, when a time interval between distance measurement operations, i.e., an emission interval of the IRED is shortened, the IRED is heated by a drive current, and this leads to a decrease in light amount or destruction of the IRED. Therefore, it is impossible to shorten a time interval between distance measurement operations for speed detection with a simple arrangement.